1. Field of the Invention
The invention relates to signal processing, and more particularly to Doppler wipe-off of signals.
2. Description of the Related Art
Before a transmitter sends a segment of data to a receiver, the transmitter modulates the signal according to a code to obtain a signal suitable for transmission. When a receiver receives the signal, the receiver must therefore remove the code from the received signal to recover the original data. The receiver must synchronize the phase of the local code with that of the remote code of the transmitter, and then the original data can be correctly recovered according to the local code with the synchronized phase. The receiver therefore must search a lot of code phase hypotheses for the correct phase of the local code before the original data are decoded.
In addition, before the transmitted signal arrives at the receiver, the signal has propagated through a transmission path and may suffer from distortion due to Doppler frequency shift in the transmission path, transmitter and receiver dynamics, or clock drift and clock bias. The receiver therefore must estimate a Doppler frequency shift of the received signal and compensate the received signal for the Doppler frequency shift before the received signal is further processed. The receiver therefore must also search a lot of Doppler shift hypotheses for the correct Doppler frequency shift of the received signal before the received signals are processed.
Assume that the incoming signal uncertainty is divided into N code phase hypotheses and M Doppler shift hypotheses. The N code phase hypotheses and M Doppler shift hypotheses forms the N×M combined hypotheses. Searching N×M combined hypotheses, however, is time consuming, delaying subsequent signal decoding and degrading system performance of the receiver. A circuit determining the Doppler shift and the code phase with a rapid speed is therefore required to improve the system performance of the receiver.